In mobile communication systems, communication processes could involve a large amount of uncertainties due to time-varying characteristics of wireless fading channels. On one hand, in order to increase the system throughput, high order modulation and low redundancy error correction coding with high transmission rate can be adopted in communications. In this way, indeed, the system throughput can be significantly increased when the wireless fading channel has an ideal Signal-to-Noise Ratio (SNR). On the other hand, in order to guarantee the reliability of communications, low order modulation and high redundancy error correction coding with low transmission rate can be adopted to provide reliable communications in a wireless deep-fading channel. However, when the SNR of the channel is high, the low transmission rate constrains the increase of the system throughput, resulting in a waste of resources. In early years of the development of mobile communication technologies, in order to combat the time-varying characteristics of wireless fading channels, people can only increase a transmit power of a transmitter and use a low-order, high-redundancy modulation and coding scheme to guarantee the communication quality in a deep-fading channel, leaving no room for system throughput increase. With the technical advancement, a technique known as adaptive coding and modulation has emerged, which can overcome the time-varying characteristics of a channel by adaptively adjusting a transmit power, a modulation and coding scheme and a length of data frame based on the channel condition, so as to achieve the best communication effect. This is the most typical link adaptation technique.
In the Long Term Evolution (LTE) system, control signals to be transmitted in uplink include Acknowledgement/Negative Acknowledgement (ACK/NACK) messages and three types of indications of Channel State Information (CSI) for downlink physical channels: Channel Quality Indication (CQI), Pre-coding Matrix Indicator (PMI), and Rank Indicator (RI).
TABLE 1ModulationCQI IndexSchemeCode Rate *1024Efficiency0out-of-range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
The CQI is an indicator of the quality of a downlink channel. As shown in Table 1 above, the CQI is represented in the 36.213 specification by an integer value ranging from 0 to 15. These integer values represent different CQI levels corresponding to respective Modulation and Coding Schemes (MCSs), as shown in Table 1.
In the above Table 1, QAM stands for Quadrature Amplitude Modulation and QPSK stands for Quadrature Phase Shift Keying, which are digital modulation schemes.
The CQI level should be selected such that, with the corresponding MCS, Physical Downlink Shared Channel (PDSCH) Transport Blocks (TBs) corresponding to the CQI can have a Block Error Ratio (BLER) lower than 0.1.
Based on a non-limiting detection interval in frequency domain and time domain, the highest CQI value a User Equipment (UE) can obtain corresponds to the highest CQI value reported in an uplink subframe n. The CQI index ranges from 1 to 15 if a particular condition is satisfied and the CQI index is 0 when the CQI index 1 does not satisfy that condition. The condition is as follows: one single PDSCH TB has an error rate lower than 0.1 when it is received; the PDSCH TB includes joint information of modulation scheme and Transport Block Size (TBS), which corresponds to a CQI index and a set of occupied downlink Physical Resource Blocks (RPBs) (i.e., CQI reference resource). Here, the highest CQI value is a maximum CQI value capable of guaranteeing a BLER lower than 0.1, which is advantageous in control of resource allocation. In general, the smaller the CQI value, the more the resources to be occupied and the better the BLER performance.
A CQI index corresponds to a joint information of TBS and modulation scheme, if such joint information for PDSCH transmission in the CQI reference resource can be signaled according to the corresponding TBS. And the effective channel coding rate, resulted from the modulation scheme, is the possible closest effective channel coding rate the CQI index can represent, where the modulation scheme is represented by the CQI index and use the joint information of TBS and modulation scheme in the reference resource. When more than one piece of joint information can generate equally close effective channel coding rate represented by the CQI, the joint information having the smallest TBS can be used.
Each CQI index corresponds to a modulation schema and a TBS. The correspondence between TBSs and the number, NPRB, of PRBs can be represented in a table. The coding rate can be calculated from the TBS and the value of NPRB.
In the LTE system, an ACK/NACK message is transmitted on Physical Uplink Control Channel (PUCCH) in PUCCH format 1/1a/1b. If a terminal (e.g., UE) needs to transmit uplink data, the data can be transmitted on Physical Uplink Shared Channel (PUSCH). The feedback of CQI, PMI or RI can be periodic or aperiodic. This can be seen in Table 2, which shows physical uplink channels corresponding to periodic and aperiodic feedbacks, respectively.
TABLE 2Periodic CQI Aperiodic CQI Scheduling ModeReport ChanelReport ChanelFrequency Non-PUCCHselective ChannelFrequency SelectivePUCCHPUSCHChannel
Here, the periodic feedbacks of CQI, PMI or RI can be transmitted on PUCCH in PUCCH format 2/2a/2b if the UE does not need to transmit any uplink data, or can be transmitted on PUSCH when the UE needs to transmit uplink data. The aperiodic feedbacks of CQI, PMI or RI can only be transmitted on PUSCH.
In LTE Release 8 standards, three physical downlink channels have been defined: Physical Control Format Indicator Channel (PCFICH), Physical Hybrid Automatic Retransmission Request Indicator Channel (PHICH) and Physical Downlink Control Channel (PDCCH). The PDCCH carries Downlink Control Information (DCI) including uplink and downlink scheduling information and uplink power control information. There are a number of DCI formats, including DCI format 0, DCI format 1, DCI format 1A, DCI format 1B, DCI format 1C, DCI format 1D, DCI format 2, DCI format 2A, DCI format 2B, DCI format 2C, DCI format 2D, DCI format 3, DCI format 3A, etc.
In the LTE, it is required to define downlink control information such as coding and modulation schemes, resource allocation positions, Hybrid Automatic Repeat request (HARQ) via downlink control signaling. The coding and modulation schemes can be determined via downlink scheduling by the base station. Alternatively, a table of modulations and TBSs has been defined in the standard. Each line in the table corresponds to an MCS index. For each MCS index, the table of modulations and TBSs defines a combination of modulation scheme and code rate, referring to the LTE 36.213 standard. In essence, an MCS index is associated with spectrum efficiency. The MCS index shall be selected with reference to the CQI value and in practice the base station needs to consider their spectrum efficiency. Once the base station determines the MCS index, it needs to determine resource allocation information, which gives the number, NPRB, of PRBs to be occupied by a downlink transmission. The LTE standard further provides a TBS table defining TBSs for each given MCS and NPRB. With these coding and demodulation parameters, the downlink coding and modulation can be performed.
In Release 10 (R10), a UE can be configured semi-statically, via higher layer signaling, as one of the following transmission modes, so as to receive PDSCH data transmissions according to an indication of PDCCH in a UE-specific search space:
Transmission Mode 1: single-antenna port, port 0;
Transmission Mode 2: transmit diversity;
Transmission Mode 3: open-loop spatial multiplexing;
Transmission Mode 4: closed-loop spatial multiplexing;
Transmission Mode 5: multi-user Multiple Input Multiple Output (MIMO);
Transmission Mode 6: closed-loop Rank=1 precoding;
Transmission Mode 7: single-antenna port, port 5;
Transmission Mode 8: dual-stream transmission, i.e., dual-stream beamforming;
Transmission Mode 9: up to 8-layer transmission; and
Transmission Mode 10: up to 8-layer transmission with COMP feature.
After Releases 8/9/10/11/12, the LTE system continues evolving towards Release 13. In the evolution of Release 13 standard, Machine Type Communications (MTC) becomes a critical topic. MTC terminals include normal MTC terminals and MTC terminals with coverage enhancement. Moreover, New Radio Access Technology (RAT), or NR, in the 5th Generation (5G) of mobile communication systems has become a hot topic.
In the related standards, modulation and coding schemes of up to 64 QAM can be supported in uplink and downlink. With the development of MTC terminals or 5G terminals, MTC terminals with coverage enhancement or 5G terminals require higher data transmission reliability, better coverage and lower data transmission rate, which cannot be fulfilled by the related standards.
There is currently no effective solution to the above problem in the related art.